Strong shocks should compress the magnetic field and lead to
high degrees of polarization of 40-70% in the radio continuum
(Beck 1982)
at the inner edges of the spiral arms (see
Section 8.4). Only in M51 are the strongest
aligned fields indeed found at the
positions of the prominent dust lanes on the inner edges of the optical
spiral arms (Figure 1).
This is best visible along the eastern arm where the aligned field
even follows the dust lane crossing the optical arm.
However, some regular fields extend far into the interarm regions.
Furthermore, the 10-30% polarization at
6 cm is
in contrast to the higher polarizations expected from shock alignment.
Hence the radio data only tell us that the regular fields in M51
are somehow coupled to the cool gas as traced by dust lanes.

The aligned fields in M81 and NGC 1566 are strongest in interarm regions
(Krause et al 1989b,
Ehle et al 1996),
whereas the total
synchrotron intensity (tracing the total field) is highest in the
optical spiral arms. Strongly aligned interarm fields
have also been detected in the outer parts of M83,
where the star formation rate is low
(Allen & Sukumar 1990).
High-resolution observations of M81
(Schoofs 1992;)
see Figure 2)
confirmed that the regular fields extend across almost the entire
interarm region, but are somewhat stronger near the inner edge
of the prominent western spiral arm,
where some dust clouds are visible. We stress that the distribution of
magnetic pitch angles exhibits a weaker arm-interarm
variation than that of the regular magnetic field strengths.
Soida et al (1996)
showed that strength and pitch angle of the
regular fields in NGC 4254 reveal much less arm-interarm variations
than expected from density-wave compression in its two major arms. They
also showed that regular fields even exist in regions of chaotic optical
pattern.

Figure 2. Polarized synchrotron intensity
(contours) and magnetic field orientation in the south-western
part of M81 (obtained by rotating the E-vectors by 90°),
observed at
6.2 cm with the VLA
(25 arcsec synthesized beam). The circle indicates the
half-power diameter of the primary beam. (From
Schoofs 1992.)